1. Field of the Invention
The present invention relates to a method for evaluating a composition ratio of a crystalline lithium niobate (LiNbO.sub.3) thin film known as a ferroelectric material by measuring the absorption edge thereof, and to an apparatus for preparing an LiNbO.sub.3 thin film capable of controlling conditions for forming an LiNbO.sub.3 thin film while monitoring the film being formed by the use of the above method.
2. Prior Arts
Among various ferroelectric oxides, LiNbO.sub.3 is particularly characterized by its high melting point, its high Curie temperature and good chemical stability. Accordingly LiNbO.sub.3 is widely used in various fields as a material for optical integrated circuits, surface elastic wave devices and the like which utilize electrooptic effects, non-linear optic effects, piezoelectric effects or the like. Further, for the purpose of making good use of the properties of this material and of minimizing the prices of developed devices and the like, development of devices using an LiNbO.sub.3 thin film is under way, and a technique for preparing an LiNbO.sub.3 thin film is needed.
An LiNbO.sub.3 thin film is formed using sputtering, ion plating or CVD method. Usually a hetero-epitaxial single-crystalline LiNbO.sub.3 film is formed on a single-crystalline substrate of .alpha.-Al.sub.2 O.sub.3, MgO, ZnO or the like, while a polycrystalline LiNbO.sub.3 film is formed on a glass- or polycrystalline ceramic substrate.
However, in the case of preparing LiNbO.sub.3 thin films as described above, it is necessary to develop a method for efficiently evaluating the stoichiometrical composition ratio of an LiNbO.sub.3 thin film. Evaluation of the stoichiometrical ratio is meant here to evaluate whether or not Li/Nb composition ratio of an LiNbO.sub.3 thin film is 1. As well, it is necessary to establish a method for preparing an LiNbO.sub.3 thin film with a good reproducibility of the stoichiometrical ratio.
In the case of forming an LiNbO.sub.3 film by sputtering or ion plating, the vapor pressure of Li is extremely high relative to that of Nb, and hence it has been difficult to establish conditions for forming uniform LiNbO.sub.3 thin films of the stoichiometrical ratio (Li/Nb=1). Therefore, a film formation test taking a lengthy period of time has been required.
The above film formation test has been carried out every time a thin film preparing apparatus or film-forming conditions are changed. For example, the composition ratio of a target is changed in the test using sputtering, while the deposition ratio of Li to Nb is changed in the test using ion plating.
Further, a composition ratio of the thus formed thin film has been measured by means of Secondary Ion Mass Spectrometer (SIMS), Auger Electron Spectrometer (AES) or the like, and hence it has taken a long period of time to analyze. These apparatus cannot avoid destructive analysis and at the same time have difficulties in analyzing insulating materials. Furthermore, they are expensive and hard to manipulate.
Accordingly, the analysis of LiNbO.sub.3 thin films has been a bottleneck in repeatedly performing the film formation test for optimizing the film-forming conditions. Hence, a method capable of evaluating a composition ratio of the thin film in a short period of time has been required.